I. Field of the Invention
This invention relates to dynamic gain adjustment of a servo control by which velocity of a member driven by a motor may be maintained in order to provide path accuracy thereto, and in particular to velocity control of separate drive motors located at distal ends of a rigid member, such as a horizontal gantry.
II. Description of the Prior Art
By way of background, one example of a machine which may advantageously use the dynamic gain adjustment feature of the present invention is a computerized tape-laying machine. Such a machine includes a horizontal gantry mounted for linear movement above the ground on a machine frame including left and right sidewalls fixedly supported on respective left and right pylons. Mounted for linear movement perpendicularly relative to the gantry is a carriage which movably supports a tape applicator head. The tape head is vertically and rotationally movable such that in cooperation with the gantry and carriage, a tape dispensing point of the tape head is movable in a plurality of rectilinear and/or rotational axes under control of a computer program by which to apply several plies of composite material or tape to a layup placed on a mold between the pylons to form aircraft parts, for example.
Movement of each member (gantry, carriage, etc.) is effected by one or more motors under control of respective servo controls. As is well known, commands from a computer control or the like to a servo control will cause the servo control to generate appropriate voltage signals to effectuate rotation of the related motor by which the member is propelled. That is, in response to appropriate commands from the computer, the selected members will thus be driven along the desired linear or rotational axis a desired distance or angle in a desired direction. By moving the members as desired, the tape applicator head will follow a desired path and cause various patterns of built-up composite layers to be placed upon the layup.
Movement of a member is effected by a motor coupled to a servo control. The computer calculates the distance the member is to be moved and, based upon predetermined feed rates, determines how far the member should move in a predetermined time or interpolation interval. The computer will utilize that information to repeatedly generate change in position commands by which to simultaneously instruct the servo control to cause the motor to move the member a particular distance during that particular interpolation or iteration interval. Thus, for example, instructing the member to move 0.01 inch every 10 ms ideally results in effecting movement of the member at a velocity of 1.0 inch/sec. The servo control will then generate a voltage signal corresponding to the desired velocity of the member, which velocity is itself correlated to the desired position commands from the computer.
More particularly, a resolver coupled to each drive motor generates a resolver signal which is utilized to indicate to the associated servo control the position of the member. Coupled between each motor and related servo control is a drive amplifier to supply motor drive currents in response to the velocity signal from the associated servo control. The motor may also provide a tachometer signal for use by the drive amplifier in a velocity feedback loop as is conventional. The servo controls will each generate a velocity command signal based upon an error signal which is derived from the actual position of the member (as indicated by the resolver signal) and the desired position thereof (as indicated by the change in position command signal from the computer). Typically, the error signal is the difference between the actual and change in position signals. The servo control will typically multiply the error signal by a constant, pre-determined gain factor signal to generate the velocity command signal. The velocity command signals are then converted in the servo control to voltage signals and coupled through an associated drive amplifier to a respective motor to cause movement of the member at a velocity correlated to the error and gain factor signals. The gain factor signal is selected so that the voltage signal corresponding to the velocity command signal will result in movement of the gantry at a predetermined velocity correlated to a predetermined error signal, e.g., 1 inch/min for one-thousandth inch error signal ( 1 inch/min per 1/1000 FE). The gain factor signal facilitates for correction of known offsets and/or relationships in the gear mechanisms, for example.
During normal operation, certain circuit components of the servo control equipment may drift, such as from temperature variations. This is especially a problem with the drive amplifiers. In some situations, such as where the tape head is to be moved over a substantial distance, drift in the drive amplifiers may cause one or more of the various members to follow paths different from the desired paths, whereby the tape head also does not follow the desired path. For example, while the computer is repeatedly instructing a servo control to cause movement of a member a specified distance, a drive amplifier is being provided voltage signals correlated to the change in position command to cause the motor to move the member at a predetermined velocity for a specified period of time. However, due to the effects of drive amplifier drift the motor may be driven by currents which vary such that the true velocity of movement of the member may be more or less than that needed to move the requisite distance to the commanded position during the predetermined interval. As a result, during any given time interval, the member may move at a velocity other than that which is desired, leading to a path error. Such an error may cause the tape pattern to vary from the desired pattern. For example, if the tape is to be cut while the tape applicator head is moving, path error may cause an incorrect length of tape to be cut.
Drift in the drive amplifiers can have further adverse consequences with particular respect to the gantry. Movement thereof is effected by a pair of motors each coupled to a respective servo control and coupled to distal ends of the gantry adjacent each supporting sidewall of the machine frame. The computer generated change in position commands are provided to each motor to move the respective distal end at the same velocity. This type of dual servo control utilized for controlling a pair of drive motors disposed at opposite ends of the gantry is referred to as a split axis control. As is well understood, each end of the gantry must move at the same velocity in order to assure that the gantry will move in a straight line. Should one end of the gantry not move at the correct velocity, the gantry will skew. Drift in the drive amplifiers associated with the ends of the gantry may not be equal in magnitude, causing one end of the gantry to be driven at a velocity not precisely matched to that of the other end. As a result of such drift, the gantry will not move uniformly and may skew. Moreover, since the ends of the gantry are often driven at velocities as high as 1800 inch/min., even minor and temporary unequal drift between the drive amplifiers could result in substantial skewing, damaging the machine.
Heretofore, excessive skew has been accommodated by slowing or stopping the gantry. Additionally, where two servo mechanisms are to be driven by the same change in position command from a computer, skew has been reduced or avoided by compensating the gain of one of the servo mechanisms in response to a difference in the actual position of both ends of the gantry as the gantry is moving. An apparatus employing such gain compensation is disclosed in U.S. Pat. No. 4,629,955, issued Dec. 16, 1986, the disclosure of which is incorporated herein by reference. While such gain compensation can reduce skew and provide path accuracy to a member which is under split axis control by the same change in position command, such an approach may not be appropriate to control members not under control of the same change in position command. Thus, in a tape laying machine, for example, where the tape applicator head is to move in a plurality of axes under control of several change in position commands (e.g., one for each axis), the gain compensation of the aforesaid patent is not applicable to ensure the overall path accuracy of the tape applicator head.